Suppose someone says:
Hey, can you help me solve this algebra problem?
What should I say if I want to use the word forget?
My confusion is between those three. I believe I can use the simple present since what I'm saying is true and it's a fact. However, [2] and [3] seem to be true as well at the same time, I can't tell why.
I would say #1 or #2 is fine:
I forget how to do algebra: Puts the act of forgetting in the present. Maybe you do generally know it, but you can't remember right now, so you can't help.
I've forgotten how to do algebra: that is, sometime between when you learned algebra and now, you've forgotten how to do it. The act of forgetting is somewhere in the past but you still haven't remembered again.
Either of these are natural ways to explain it. Personally, I'd only use the simple past — I forgot — if it's something I forgot in the past, but also something that I normally know or have remembered since then:
I worked with Mark for five years. I'm so embarassed that I forgot his name when I went to introduce him to someone else.
I practiced the dialog from The Phantom of the Opera for weeks, but I got stagefright & I forgot my lines as soon as I got up on stage.
They all are correct, but the meaning is a bit different.
"I forget how to do algebra" means that there's some regularity to that; that you need to do some algebra from time to time, and every time you can't remember how to do it.
"I've forgotten how to do algebra" means that you have forgotten it some time ago. For example, you last did some algebra in school, and ten years passed since then, and you haven't done any algebra since then, so you have forgotten how to do it.
"I forgot how to do algebra" is the simplest one, it doesn't say anything about how frequent it happens, or how long it's been since you knew. Just the simple fact that you knew, but right now you don't.
